In radiography the interior of objects is reproduced by means of penetrating radiation, which is high energy radiation also known as ionizing radiation belonging to the class of X-rays, gamma-rays and high-energy elementary particle radiation, e.g. beta-rays, electron beam or neutron radiation. For the conversion of penetrating radiation into visible light and/or ultraviolet radiation luminescent substances, called phosphors, are used.
In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted imagewise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
In practice the light emitted imagewise by said screen irradiates a contacting photographic silver halide emulsion layer film which after exposure is developed to form therein a silver image in conformity with the X-ray image.
More recently as described e.g. in U.S. Pat. No. 3,859,527 an X-ray recording system has been developed wherein photostimulable storage phosphors are used having in addition to their immediate light emission (prompt emission) on X-ray irradiation the property to store temporarily a large part of the X-ray energy. Said energy is set free by photostimulation in the form of fluorescent light different in wavelength from the light used in the photostimulation. In said X-ray recording system the light emitted on photostimulation is detected photoelectronically and transformed into sequential electrical signals.
The basic constituents of such X-ray imaging system operating with a photostimulable storage phosphor are an imaging sensor containing said phosphor in particulate form normally in a plate or panel, which temporarily stores the X-ray energy pattern, a scanning laser beam for photostimulation, a photoelectronic light detector providing analogue signals that are converted subsequently into digital time-series signals, normally a digital image processor which manipulates the image digitally, a signal recorder, e.g. magnetic disk or tape, and an image recorder for modulated light exposure of a photographic film or an electronic signal display unit, e.g. cathode-ray tube. A survey of lasers useful in the read-out of photostimulable latent fluorescent images is given in the periodical Research Disclosure December 1989.
Of special interest in the application of said image recording and reproducing method are particular barium fluorohalide phosphors identified in U.S. Pat. No. 4,239,968. The light output of these phosphors upon stimulation with helium-neon laser beam (633 nm) is compared with the stimulated light output of SrS:0.0001Eu.0.0001Sm photostimulable phosphor described in U.S. Pat. No. 3,859,527, the basic patent in the field of radiography operating with photostimulation of storage phosphors.
According to U.S. Pat. No. 4,239,968 a method is claimed for recording and reproducing a radiation image comprising the steps of (i) causing a visible ray- or infrared ray-stimulable phosphor to absorb a radiation passing through an object, (ii) stimulating said phosphor with stimulation rays selected from visible rays and infrared rays to release the energy of the radiation stored therein as fluorescent light, characterized in that said phosphor is at least one phosphor selected from the group of alkaline earth metal fluorohalide phosphors represented by the formula: EQU (Ba.sub.1-x M.sub.x.sup.II)FX:yA
wherein:
M.sup.II is one or more of Mg, Ca, Sr, Zn and Cd; PA1 X is one or more of Br, Cl or I; PA1 A is at least one member of the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er; and PA1 x is in the range 0&lt;.times.&lt;0.6 and y is in the range 0&lt;y&lt;0.2, and that the wavelength of said stimulating rays is not less than 500 nm. PA1 x is a number satisfying 0&lt;.times.&lt;10.sup.-1, and PA1 a is a number satisfying 0&lt;a&lt;0.2. PA1 X is at least one member selected from the group Consisting of Cl and I; PA1 x is in the range 0&lt;.times.&lt;0.15; PA1 a is in the range 0.70&lt;a &lt;0.96; PA1 b is in the range 0&lt;b &lt;0.15; PA1 z is in the range 10.sup.-7 &lt;z&lt;0.15, and PA1 X is at least one halogen selected from the group consisting of Cl and I, PA1 M.sup.I is at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; PA1 M.sup.II is at least one alkaline earth metal selected from the group consisting of Ca and Mg; PA1 M.sup.III is at least one metal selected from the group consisting of Al, Ga, In, Tl, Sb, Bi, Y or a trivalent lanthanide, e.g. La, Ce, Pro Nd, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; PA1 a is a number satisfying the conditions of 0.85&lt;a&lt;0.96 when x is 0.17&lt;x&lt;0.55 and 0.90&lt;a&lt;0.96 when x is 0.12&lt;x&lt;0.17; PA1 y is in the range 0&lt;y&lt;10.sup.-1 ; PA1 b is in the range 0&lt;b&lt;0.15; PA1 p is in the range 0&lt;p&lt;0.3; PA1 q is in the range 0&lt;q&lt;0.1; PA1 z is in the range 10.sup.-6 &lt;z&lt;10.sup.-2 ; PA1 m is in the range 10hu -5&lt;m&lt;10.sup.-1, and PA1 A is Eu.sup.2+. PA1 In preferred phosphors according to said empirical formula (I) p is in the range of 10.sup.-4 &lt;p&lt;10.sup.-1, and the preferred alkali metal for shifting the maximum of the stimulation spectrum of the phosphor to the shorter wavelengths in combination with samarium is Na or Rb. PA1 (1) barium fluoride; PA1 (2) ammonium bromide; PA1 (3) optionally barium halide (except barium fluoride), PA1 (4) an alkali metal compound, e.g. lithium fluoride, lithium chloride, lithium bromide, lithium iodide, sodium fluoride, sodium chloride, sodium bromide, potassium fluoride, rubidium fluoride, cesium fluoride, lithium hydroxide or oxide or lithium carbonate, preference being given to sodium bromide or rubidium fluoride; PA1 (5) a strontium halide, optionally in admixture with a calcium and/or magnesium halide; PA1 (6) optionally a M.sup.III compound, e.g. halide or oxide wherein M.sup.III has the definition given above, preferably M.sup.III is Gd; PA1 (7) at least one A containing compound selected from the group consisting of europium halide, europium oxide, europium nitrate and europium sulphate, preferably EuF.sub.3 that is reduced to yield Eu.sup.2+ ions during firing. PA1 (8) a samarium compound, e.g. halide or oxide. PA1 --a raw mix of 0.86 mol of BaF.sub.2, 0.985 mol of NH.sub.4 Br and 0.001 mol of EuF.sub.3 was prepared. To that mix small amounts (in the range of 0.1 to 1 wt to the total solids) of Sm.sub.2 O.sub.3 were added. The thus obtained raw mix was then fired at a temperature between 700.degree. and 1000 .degree. C. in a reducing atmosphere. The firing lasted at least 3 h but may proceed up to 10 h. PA1 (i) causing a visible radiation stimulable phosphor to absorb penetrating radiation having passed through an object or emitted by an object and to store energy of said penetrating radiation in said phosphor, which is within the scope of the above defined empirical formula (I), PA1 (ii) stimulating said phosphor with visible radiation in the wavelength range from 440 nm to 550 nm, preferably in the wavelength range of 480 to 540 nm, to release energy stored in said phosphor as fluorescent light differing in wavelength range from the stimulating light, and PA1 (iii) detecting said fluorescent light preferably after substantial separation by filter means from the stimulating light.
In FIG. 3 of said U.S. Patent a graph shows the relationship between the wavelength of the stimulation rays and the luminance of the stimulated light, i.e. the stimulation spectrum wherefrom can be learned that said kind of phosphor has high photostimulation sensitivity to stimulation light of a He-Ne laser beam (633 nm) but poor photostimulability below 500 nm. The stimulated light (fluorescent light) is situated in the wavelength range of 350 to 450 nm with a peak at about 390 nm (ref. the periodical Radiology, Sept. 1983, p.834.).
It can be learned from said U.S. Pat. No. 4,239,968 that it is desirable to use a visible ray-stimulable phosphor rather than an infra-red ray-stimulable phosphor because the traps of an infra-red-stimulable phosphor are shallower than these of the visible ray-stimulable phosphor and accordingly the radiation image storage panel comprising the infra-red ray-stimulable phosphor exhibits a relatively rapid dark-decay (fading). Taking into account image fading read-out has to proceed relatively soon after the image-wise exposure to penetrating radiation and the read-out time (scanning time) has to be kept fairly short. Indeed, as explained in said U.S. Patent when the panel comprising an infra-red ray-stimulable phosphor is scanned with infra-red rays, and the fluorescent light emitted therefrom is processed electrically, a certain period of time is required to scan the overall face of the panel, and accordingly, there is the possibility that a difference arises between the initial output and the final output even though the initial portion and the final portion of the panel absorb the same amount of radiation beforehand.
For solving the problem described above it is desirable to use a photostimulable storage phosphor which has traps as deep as possible to avoid fading and to use for emptying said traps light rays having substantially higher photon energy (shorter wavelength) than the usual He-Ne laser beam of 633 nm.
Taking into account the objective of reducing the image-fading and the fact that the fluorescent light emission of barium fluoride-halide phosphors is situated at about 390 nm and is practically nihil at 450 nm preference is given to such phosphors having a stimulation maximum at about 500 nm which is still sufficiently remote from the emission spectrum of their fluorescent (stimulated) light in order to allow a good separation by optical filter means of the stimulated light from the stimulating light. The filter means absorbs or rejects the stimulating light and prevents it from entering the detector means, e.g. a photomultiplier tube having a photo-electron emission sensitivity matching the wavelength range of the stimulated light.
A further advantage of photostimulation with shorter wavelength light in comparison with the commonly used 633 nm He-Ne laser beam is an improvement in image-sharpness because shorter wavelength light of e.g. 500 nm and shorter is less diffracted in a phosphor panel containing in a binder the dispersed phosphor acting as a diffraction grating.
Bearing in mind the above, attempts have been made to formulate phosphor compositions showing a stimulation spectrum in which the emission intensity at the stimulation wavelength of 500 nm is higher than the emission intensity at the stimulation wavelength of 600 nm. A suitable phosphor for said purpose is described in U.S. Pat. No. 4,535,237 in the form of a divalent europium activated barium fluorobromide phosphor having the bromine-containing portion stoichiometrically in excess of the fluorine.
A divalent europium activated barium fluorobromide phosphor prepared for said purpose is obtained by using predetermined amounts of barium fluoride and a compound of trivalent europium, and barium halide (except for barium fluoride) in an amount more than the stoichiometric amount. The firing as explained in the Example i proceeds in a reducing atmosphere to convert Eu.sup.3+ into Eu.sup.2+.
According to claim 1 of said U.S. Pat. No. 4,535,237 the photostimulation of the phosphor with its higher emission intensity by stimulation at 500 nm proceeds with light in the wavelength range of 550 to 800 nm.
In U.S. Pat. No. 4,948,696 a divalent europium activated complex halide phosphor is described represented by the formula: EQU BaFX.multidot.xNaX':aEu.sup.2+
wherein X and X' each designate at least one of el, Br and I,
Said phosphor, actually by the definition of X, X' and "x" being likewise a phosphor having the halides other than fluorine stoichiometrically in excess of the fluorine, is claimed for use in radiography wherein the phosphor after its X-ray exposure is photostimulated with light in the wavelength range of 450 to 1,100 nm. According to the stimulation spectrum given of a particular phosphor in FIG. 1 in said lastmentioned U.S. Patent the stimulation peak is above 600 nm and photostimulability drops considerably below 500 nm.
In European patent specification 0 021 342 (see also U.S. Pat. No. 4,512,911) a rare earth element activated complex halide phosphor is described of which the luminance of light emitted upon photostimulation is enhanced by incorporating in the phosphor at least one fluoride selected from the group consisting of specific alkali metal fluorides, specific divalent metal fluorides and specific trivalent metal fluorides in a proper amount as the constituent of the host material of the phosphor which is represented by the formula: EQU Ba F.sub.2 .multidot.a BaX.sub.2 .multidot.bMgF.sub.2 .multidot.cMe.sup.I F.multidot.dMe.sup.II F.sub.2 .multidot.eMe.sup.III F.sub.3 :fLn
wherein X is at least one halogen selected from the group consisting of chlorine, bromine and iodine, Me.sup.I is at least one alkali metal selected from the group consisting of lithium and sodium, Me.sup.II is at least one divatent metal selected from the group consisting of beryllium, calcium and strontium, Me.sup.III is at least one trivatent metal selected from the group consisting of aluminium, gallium, yttrium and lanthanum, Ln is at least one rare earth element selected from the group consisting of europium, cerium and terbium, and a, b, c, d, e and f are numbers satisfying the conditions of 0.90&lt;a&lt;1.05, 0&lt;b&lt;1.2, 0&lt;c&lt;0.9, 0&lt;d&lt;1.2, 0&lt;e&lt;0.03,100.sup.-6 &lt;f&lt;0.03 and c+d+e not equal to zero.
The phosphors according to said lastmentioned formula are claimed to emit light of higher luminance than the conventional rare earth element activated divalent metal fluorohalide phosphor when stimulated by light of wavelength ranging from 450 to 800 nm after exposure to ionizing radiation such as X-rays. A stimulation spectrum of said phosphors has not been given, the measurement of luminance by photostimulation was performed with light of 630 nm which was obtained by causing the light emitted by a xenon lamp in a spectroscope to pass through a diffraction grating.
In published European patent applications (EP-A) 0 345 903, 0 345 904 and 0 345 905 (see also U.S. patent application Ser. Nos. 07/426,841, 07/426,895, 07/426,896 and 07/426,897) barium fluorohalide phosphors are mentioned that are not within the scope of the above mentioned empirical formula of said EP-A 0 021 342 and wherein the high yield of fluorescent light on photostimulation is the result of the presence of strontium and of fluorine stoichiometrically in larger atom % than bromine taken alone or bromine combined with chlorine and/or iodine. The presence of Sr together with a stoichiometric excess of fluoride with respect to the other halides at concentrations outside the preferred concentration ranges stipulated in claim 21 of said EP-A 0 021 342 surprisingly brings about a substantial increase in the X-ray conversion efficiency on photostimulation with He-Ne (633 nm) laser beam as illustrated e.g. in FIG. 6 of published EP-A 0 345 903 and FIG. 3 of published EP-A 0 345 904.
In published EP-A 0 345 905 a rare earth metal doped barium strontium fluoride phosphor is claimed characterized by the following empirical formula: EQU Ba.sub.1-x Sr.sub.x F.sub.2-a-b Br.sub.a X.sub.b :zA
wherein:
A is Eu.sup.2+ or Eu.sup.2+ together with one or more co-dopants selected from the group consisting of Eu.sup.3+, Tm, Dy, Pr, Ho, Nd, Yb, Er, Gd and Lu, and wherein fluorine is present stoichiometrically in said phosphor in a larger atom % than bromine taken alone or bromine combined with chlorine and/or iodine. From FIG. 3 of said lastmentioned published EP-A can be learned that that the gadolinium co-doped phosphor prepared according to INVENTION EXAMPLE 1 described therein is characterized by a stimulation spectrum having a maximum below 500 nm. The gadolinium co-dopant has been introduced in the phosphor firing mixture as GdF.sub.3 in the presence of EuF.sub.3, BaF.sub.2 and SrF.sub.2. An amount of NH.sub.4 Br has been used in a substoichiometric quantity (94.2% of the stoichiometric amount) to obtain a phosphor with fluorine stoichiometrically in excess with respect to bromine.
During the firing of the barium fluoride with ammonium bromide some of the ammonium bromide sublimates (542.degree. C.) so that not all the bromine of the raw material mixture is built-in in the phosphor structure.